JS 类的继承改进方法

  function extend(subclass,superclass){
      function create(proto, c) {
          function F() { }
          F.prototype = proto;
          var o = new F();
          o.constructor = c;
          return o;
      }

　　var superObj = create(superclass.prototype,subclass); //new superclass();
　　subclass.prototype = superObj;
    superObj.constructor = subclass;//矫正 constructor属性
　　subclass.superclass = create(superclass.prototype,superclass);//new superclass();
　　return subclass;
}